1. Field of Invention
The present invention is directed to a stage assembly for moving a device. More specifically, the present invention is directed to a stage assembly including a reaction mass assembly and a system and method for resetting the reaction mass assembly. The present invention also relates to an exposure apparatus and method, and more particularly to an exposure apparatus and method for transferring a pattern onto a substrate by irradiation of an exposure beam.
2. Description of Related Art
Various types of exposure apparatus are conventionally used in photolithographic processes for manufacturing semiconductor devices, liquid crystal display devices, and the like. In recent years, a step-and-repeat reduction projection exposure apparatus (a so-called “stepper”), a step-and-scan scan-exposure apparatus (a so-called “scanning stepper”), and the like have been widely used.
Exposure apparatuses are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that retains a reticle, a lens assembly and a wafer stage assembly that retains a semiconductor wafer. The reticle stage assembly and the wafer stage assembly are supported above a ground with an apparatus frame.
Typically, the wafer stage assembly includes a wafer stage base, a wafer stage that retains the wafer, and a wafer stage mover assembly that precisely positions the wafer stage and the wafer. Somewhat similarly, the reticle stage assembly includes a reticle stage base, a reticle stage that retains the reticle, and a reticle stage mover assembly that precisely positions the reticle stage and the reticle. The size of the images transferred onto the wafer from the reticle is extremely small. Accordingly, the precise relative positioning of the wafer and the reticle is critical to the manufacturing of high density, semiconductor wafers.
Unfortunately, the wafer stage mover assembly generates reaction forces that can vibrate the wafer stage base and the apparatus frame. The vibration influences the position of the wafer stage base, the wafer stage, and the wafer. As a result thereof, the vibration can cause an alignment error between the reticle and the wafer. This reduces the accuracy of positioning of the wafer relative to the reticle and degrades the accuracy of the exposure apparatus. Reaction forces produced due to driving of the wafer stage is mechanically caused to escape to the floor (the ground) by a frame member placed on a base (e.g., a floor surface or a base plate of the apparatus) which is vibration-isolated from the stage, as disclosed in, for example, U.S. Pat. No. 5,528,118.
In the case of the scanning stepper, a reticle stage as well as a wafer stage needs to be driven in a predetermined scanning direction by a linear motor or the like. In order to absorb reaction forces produced due to driving of the reticle stage, a countermass mechanism for one scanning direction, which functions based on the law of conservation of momentum, is typically adopted (see, for example, U.S. patent application Ser. No. 09/260,544). The reaction force produced due to driving of the reticle stage can also be mechanically transferred to the base, that is, the floor (the ground) by using a frame member (see, for example, U.S. Pat. No. 5,874,820).
In conventional projection exposure apparatus, the reaction force of the stage to be transferred to the base is damped by a vibration-isolating device, such as an anti-vibration table, so as to reduce vibration of a projection optical system (projection lens) and vibration of the stage transmitted via the base due to the reaction force. Although the reaction force is damped by being transferred to the base, a nontrivial amount of vibration, from the viewpoint of the level required under current micro-fabrication requirements, is given to the projection optical system and to the stage. Such vibration resulting from the reaction force deteriorates exposure accuracy of a scanning stepper that performs an exposure operation while scanning a stage (and a wafer or a reticle).
While transmission of reaction force can be substantially completely prevented by absorbing the reaction force by the countermass mechanism, the conventional countermass mechanism employs a countermass that moves in a direction opposite from the driving direction of a stage by a distance proportional to the driving distance of the stage. For this reason, the stroke of the countermass must be set in accordance with (in proportion to) the total stroke of the stage, which increases the size of the exposure apparatus.
In light of the above, one object of the present invention is to provide a stage assembly that precisely positions a device. Another object is to provide a stage assembly that minimizes the influence of the reaction forces of the stage mover assembly upon the position of the stage, the stage base, and the apparatus frame. Still another object is to provide a stage assembly having an improved reaction mass assembly. Another object is to provide an improved system and method for resetting the position of a reaction mass assembly. Yet another object is to provide an exposure apparatus capable of manufacturing precision devices such as high density, semiconductor wafers.